3 km from Widows Row, Ireland

Flush bracket bench mark on Slieve Donard Triangulation Pillar J3527 : Slieve Donard Triangulation Pillar. The number of the bracket is OSNIBM 3087. The pillar is 853m above sea level but the bracket itself does not appear to have been levelled.

The re-triangulation of Northern Ireland by the Ordnance Survey Northern Ireland began in 1950. This was the first complete survey of Northern Ireland which included observations with the new primary triangulation of the country, its connection with the Republic of Ireland and the cross-channel connection between Ireland and Great Britain. This began by OSNI establishing a series of triangulation stations throughout the country. Almost all of these stations were topped by trig pillars and 80, mainly primary and secondary pillars, had been constructed by October 1949. Measurements between primary stations began in 1950 and measurements for these and the secondary stations were completed by July 1956. The construction and measurements for tertiary stations were completed later (probably no later than the mid 1960s). Only two stations are not topped by pillars - Lighthouse Island, marked by a brass rivet, and Ardglass, which utilised the top of a high stone folly. The older triangulation stations on the Lough Foyle Base Line were also re-surveyed as part of this process. A majority of the stations are still extant today but a few have been removed or destroyed.

Bench marks were historically used to record the height above sea level of a location as surveyed against the Mean Sea Level data (taken at Clarendon Dock, Belfast, for Northern Ireland data, Newlyn in Cornwall for data in Great Britain and Portmoor Pier, Malin Head, for data relating to the Republic of Ireland). They were used as part of a greater surveying network by the UK Ordnance Survey, Ordnance Survey Northern Ireland (OSNI) and the Ordnance Survey Ireland (OSI). If the exact height of one bench mark is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. In this way hundreds of thousands of bench marks were sited all around the UK & Ireland from the mid 19th to late 20th centuries. There are several distinct types of bench mark:

Bench marks are commonly found on older buildings or other semi-permanent features such as stone bridges or walls. Due to updated mapping techniques and technological advances such as GPS, bench marks are no longer maintained. Many are still in existence and the markers will probably remain until they are eventually destroyed by redevelopment or erosion.

The Mourne Wall is a 22 mile long wall in the Mourne Mountains. It was built between 1904 and 1922 by the Belfast Water Commissioners to enclose their water catchment areas in the Mournes and protect the area from the effects of cattle and sheep on the water course. The wall is predominately constructed from local granite stone using traditional dry stone walling techniques; on average the wall is about 1.5 metres high and 0.8 to 0.9 metres thick. It is not uniform in construction along the entire length - the 'classic' granite wall is only to be found north of Carn mountain and Long Seefin with particularly impressive sections on Slieve Commedagh and Slieve Donard; elsewhere the wall largely resembles dry stone walls found elsewhere in the Mournes and south County Down. In places, such as Slieve Muck, the wall is not constructed of granite at all.